Polyvinyl Chloride Microplastics Leach Phthalates into the Aquatic Environment over Decades

Phthalic acid esters (phthalates) have been detected everywhere in the environment, but data on leaching kinetics and the governing mass transfer process into aqueous systems remain largely unknown. In this study, we experimentally determined time-dependent leaching curves for three phthalates di(2-ethylhexyl) phthalate, di(2-ethylhexyl) terephthalate, and diisononyl phthalate from polyvinyl chloride (PVC) microplastics and thereby enabled a better understanding of their leaching kinetics. This is essential for exposure assessment and to predict microplastic-bound environmental concentrations of phthalates. Leaching curves were analyzed using models for intraparticle diffusion (IPD) and aqueous boundary layer diffusion (ABLD). We show that ABLD is the governing diffusion process for the continuous leaching of phthalates because phthalates are very hydrophobic (partitioning coefficients between PVC and water log KPVC/W were higher than 8.6), slowing down the diffusion through the ABL. Also, the diffusion coefficient in the polymer DPVC is relatively high (∼8 × 10–14 m2 s–1) and thus enhances IPD. Desorption half-lives of the studied PVC microplastics are greater than 500 years but can be strongly influenced by environmental factors. By combining leaching experiments and modeling, our results reveal that PVC microplastics are a long-term source of phthalates in the environment.


Instruments
The centrifuge was a CR422 (Jouan GmbH, Unterhachingen, Germany). The pH was measured using a Multi 9620 IDS multi-parameter benchtop meter (WTW, Weilheim,

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Germany). For the solid phase extraction, an accelerated solvent extractor (ASE 200, Thermo Fisher Scientific, Waltham, US) was used. Extracts were concentrated using a laboratory evaporator (Barkey vapotherm basic mobil I, Leopoldshöhe, Germany). Phthalates were quantified using a GC 8890 coupled to a triple quadrupole MS 7000D (both Agilent

S2.2 Glass transition temperature, density and molar mass distribution
The glass transition temperature of the PVC-microplastics was determined using Differential Scanning Calorimetry (DSC). For the analysis 8.5 mg of PVC-microplastics were heated to 150 °C at 20 °C min -1 . Measurements were made using a Q2000 V24.10 Build 122 (TA Instruments, New Castle, US). The density of the PVC-microplastics was measured using gas pycnometry (at 20 °C) following DIN standard. 1 The molar mass distribution of the PVC-microplastics was studied using Gas Permeation Chromatography (GPC). Therefore, 2 mg mL -1 of the respective PVC-microplastic were dissolved in the eluent. The resulting solutions were filtered through 0.2 µm syringe filters (Sartorius, Göttingen, Germany) prior to the sample injection. The injection volume was 100 µL. Samples were measured using a modular GPC system with two PLgel 10 μm MIXED-B, 7.5 mm x 300 mm columns (Agilent Technologies, Santa Clara, US) and a refractive index detector (Agilent 1100, Agilent Technologies, Santa Clara, US). The column temperature was set to 35 °C and with a flow rate of 1 mL min -1 . A mixture of tetrahydrofuran with 0.1 % trifluoroacetic acid served as the mobile phase. From the GPC spectra the weight-averaged molecular weight (Mw) and the number-averaged molecular weight (Mn) were determined ( Table 1).

S2.3 Phthalate content
The phthalate content of the PVC-microplastics was determined following the standard operation procedure. 2 Briefly, 50 mg of PVC-microplastics were weighed into 40 mL glass vials and dissolved in 5 mL of tetrahydrofuran. The vials were placed on a horizontal shaker at 125 rpm for 30 min to guarantee complete dissolution of the sample. 10 mL of n-hexane were added and the vials were again placed on the horizontal shaker. Afterwards, the vials were centrifuged at 1000 G at 20 °C for 30 min to allow the polymer to precipitate. Since a high phthalate content was expected for all PVC-microplastics, samples were diluted by spiking 25 µL of the clear supernatant into 2.5 mL of n-hexane. 980 µL of the diluted sample were spiked S-5 with 20 µL of deuterated DEHP-d4 standard (corresponding to 1 µg DEHP-d4) for the quantification of the phthalates. The samples were measured using GC-MS/MS (see S1).

S3 Sequential leaching experiment
Instantaneous leaching of phthalates from PVC-microplastics into aqueous solutions was investigated by conducting sequential leaching experiments with DEHP38%. Either 1, 2, or 3 PVC-microplastic pellets (30.7 ± 1.26 mg each) were added to 40 mL aqueous solution. The experiments were conducted in triplicates. The vials were placed on a horizontal shaker at 125 rpm, at 20 °C in dark, to prevent DEHP from photo-oxidation of phthalates. After 20 h, the PVC-microplastics were taken out with tweezers. The PVC-microplastics were air-dried and then added to 40 mL of fresh aqueous solution for 20 h. The subsequent sampling of the microplastics was done as described above. This procedure was repeated six times. To quantify DEHP in the aqueous phase, 1 µg of DEHP-d4 was added. The aqueous phase was extracted using liquid-liquid extraction using n-hexane as solvent. The hexane-extracts were concentrated to 100 µL and measured using GC-MS/MS (see S1). Since the PVC-microplastics used for the sequential leaching experiment were slightly heavier (thus had a larger surface area) than the ones used for the continuous leaching experiment, slightly higher masses leaching instantaneously were expected here.

S4 The aqueous boundary layer thickness
The aqueous boundary layer thickness δ depends on viscous forces in the solution. Though

S5 Mass transfer Biot number
The mass transfer Biot number BiM (-) compares the ratio of the diffusion in the aqueous phase in contrast to the diffusion in the microplastics and can be used to evaluate the relative importance of ABLD and IPD to the overall diffusion process. BiM can be expressed as: 4

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Bi M = k * L Ch D PVC * K PVC/W For spheres the characteristic length LCh is the particle radius r (m), DPVC (m 2 s -1 ) is the diffusion coefficient in PVC and k (m s -1 ) is the external mass transfer coefficient. Substituting for k through Daq δ -1 3 , BiM can be calculated by: While for BiM > 40 IPD and for BiM < 0.1 ABLD is the limiting diffusion process, for 0.1 < BiM < 40, both processes contribute to the overall diffusion process. 4 BiM for the PVC-microplastics used in this study were calculated using the fitted or calculated values for Daq, δ and KPVC/W (

S7 Dependence of D PVC on the phthalate content of the PVC-microplastics
The  Reported DPVC by Griffiths et al. 6 (black circles) are fitted using an exponential function (dashed line). S-9

S8 Long-term prediction of the continuous leaching
By solving Equation 6 for time, the specific desorption time for ABLD limited desorption processes can be expressed as follows: By inserting fdesorbed of 0.5, the desorption half-lives are calculated by: The long-term leaching of the PVC-microplastics containing similar amounts of phthalates (DEHP38%, DOTP35% and DINP39%) can be visualized by: Predicted leaching data are based on ABLD model and calculated for 10 7 d, i.e., 27,397 years. S-10

S9 Influence of particle size on the leaching process
In the aquatic environment plastic particles of different sizes can be found.

S11 DOC-facilitated transport through the aqueous boundary layer
To consider facilitated transport of phthalate-DOC complexes through the ABL, the mass transfer coefficient k (= Daq δ -1 ) in Equation S8 was substituted for: 10 k DOC−inclusive = k + k DOC * K DOC/W * cDOC where kDOC-inclusive (m s -1 ) is the mass transfer coefficient including DOC-facilitated transport and kDOC is the mass transfer coefficient of phthalate-DOC complexes (m s -1 ) and can be approximated by 0.02* k. 11